In conventional control, feedback control of PID control has been generally used. In the PID control, a control output is always determined with a delay from a phenomenon, and thus, when each control gain of PID is increased in an attempt to increase a control speed, the control no longer catches up with a phenomenon, and thus, the control becomes unstable. Particularly, when a mechanical damping force of a controlled object remarkably reduces, the control easily becomes unstable, and the control may diverge in some cases. As a method for determining each control gain of the PID control in order to avoid the instability of the control, a control theory, such as an H∞, which can ensure the stability of control is applied. However, under a restriction of the PID control, overshoot and control delay occur due to load fluctuation.
Also in the PID control, when sliding mode control is used, influence of load fluctuation can be theoretically eliminated by switching control gains according to a control state. However, when a control period is lengthened, the control keeps oscillating and no longer converges. Therefore, in order to completely eliminate the influence of the load fluctuation, control gains need to be switched at an infinitely high speed, and high-speed control is needed to an extent that can be said to be infinitely fast with respect to a phenomenon. Furthermore, since adjustment of each control gain, such as PID, is needed, and quality of adjustment of the control gain determines quality of control, the adjustment of the control gain becomes a very important factor.
In addition, these control theories are the theories to make up for faults of the PID control, and are not techniques constructed for a purpose of control to “stop a controlled object at a target position in a shortest time”. Therefore, for this simple purpose, it can be said that time optimal control is a control method more suitable for the purpose rather than the PID control.
Simplest time optimal control is the control to stop a controlled object at a target position by accelerating the controlled object by means of a maximum thrust force up to a halfway point to the target position, and by decelerating it at a maximum deceleration in the rest of the way. Since this output pattern is determined before the start of the control, the time optimal control can be said to be feedforward control.
In other words, the time optimal control is a control method for moving a controlled object by a maximum driving force of an actuator and stopping by a maximum braking force, and is the control that can stop the controlled object at a target in the shortest time in theory. That is, the time optimal control is the control method that perfectly meets a purpose of control to “stop the controlled object at the target position in the shortest time”.
For example, as described in Japanese patent application Kokai publication No. 2000-94371, as a control device using the time optimal control, a time optimal control device of a robot is proposed, which includes: a control unit configured to control a servomotor; a correspondence relation storage unit configured to store a relation between a controlled variable on the basis of a value at the time of no load and a load weight; a load estimation calculation unit; an acceleration/deceleration constant determination unit configured to determine an acceleration/deceleration constant based on workpiece information calculated by the load estimation calculation unit; and a command creation unit configured to create a command to be delivered to the servo control unit using the determined acceleration/deceleration constant, and which lengthens an acceleration time when grasping a workpiece and shortens the acceleration time when not grasping the workpiece.
However, while the time optimal control is the ideal control that can control with the shortest time in theory, it is an open control in which an output pattern is determined by taking into consideration an initial velocity, a maximum acceleration, and a maximum deceleration, and since there is no feedback element, there is such a problem that no modification method can be used when the target and the controlled variable do not agree, and that it is difficult to make the target and the controlled variable accurately agree with each other, and thus the time optimal control is rarely employed in actual control.